Selective CDK7 Inhibition Suppresses Cell Cycle Progression and MYC Signaling While Enhancing Apoptosis in Therapy-resistant Estrogen Receptor-positive Breast Cancer

PURPOSE

Resistance to endocrine therapy (ET) and CDK4/6 inhibitors (CDK4/6i) is a clinical challenge in estrogen receptor (ER)-positive (ER+) breast cancer. Cyclin-dependent kinase 7 (CDK7) is a candidate target in endocrine-resistant ER+ breast cancer models and selective CDK7 inhibitors (CDK7i) are in clinical development for the treatment of ER+ breast cancer. Nonetheless, the precise mechanisms responsible for the activity of CDK7i in ER+ breast cancer remain elusive. Herein, we sought to unravel these mechanisms.

EXPERIMENTAL DESIGN

We conducted multi-omic analyses in ER+ breast cancer models in vitro and in vivo, including models with different genetic backgrounds. We also performed genome-wide CRISPR/Cas9 knockout screens to identify potential therapeutic vulnerabilities in CDK4/6i-resistant models.

RESULTS

We found that the on-target antitumor effects of CDK7 inhibition in ER+ breast cancer are in part p53 dependent, and involve cell cycle inhibition and suppression of c-Myc. Moreover, CDK7 inhibition exhibited cytotoxic effects, distinctive from the cytostatic nature of ET and CDK4/6i. CDK7 inhibition resulted in suppression of ER phosphorylation at S118; however, long-term CDK7 inhibition resulted in increased ER signaling, supporting the combination of ET with a CDK7i. Finally, genome-wide CRISPR/Cas9 knockout screens identified CDK7 and MYC signaling as putative vulnerabilities in CDK4/6i resistance, and CDK7 inhibition effectively inhibited CDK4/6i-resistant models.

CONCLUSIONS

Taken together, these findings support the clinical investigation of selective CDK7 inhibition combined with ET to overcome treatment resistance in ER+ breast cancer. In addition, our study highlights the potential of increased c-Myc activity and intact p53 as predictors of sensitivity to CDK7i-based treatments.

CDK4/6 inhibitor resistance in estrogen receptor positive breast cancer, a 2023 perspective

CDK4/6 inhibitors have become game-changers in the treatment of estrogen receptor-positive (ER+) breast cancer, and in combination with endocrine therapy are the standard of care first-line treatment for ER+/HER2-negative advanced breast cancer. Although CDK4/6 inhibitors prolong survival for these patients, resistance is inevitable and there is currently no clear standard next-line treatment. There is an urgent unmet need to dissect the mechanisms which drive intrinsic and acquired resistance to CDK4/6 inhibitors and endocrine therapy to guide the subsequent therapeutic decisions. We will review the insights gained from preclinical studies and clinical cohorts into the diverse mechanisms of CDK4/6 inhibitor action and resistance, and highlight potential therapeutic strategies in the context of CDK4/6 inhibitor resistance.

Abstract P4-08-16: Selective Androgen Receptor Modulators in combination with CDK4/6 inhibitors demonstrate anti-cancer activity in preclinical treatment resistant ER+AR+ breast cancer models

Background: The Androgen Receptor (AR) is expressed in up to 90% of all ER+ breast cancers and has been associated with better patient outcome. While androgens were used at a high dose as an anticancer therapy historically, this was discontinued with the advent of Tamoxifen due to virilising effects. Non-steroidal, tissue selective AR modulators (SARMs) represent an attractive alternative, offering a targeted approach to AR activation. Recent compelling pre-clinical data has established that the AR is a tumour suppressor in ER+ breast cancers and that AR activation with a natural androgen or a SARM suppressed ER-driven tumour growth, in preclinical models of endocrine-sensitive and -resistant ER+ breast cancer. Here, we evaluate the efficacy of a SARM (enobosarm) and a natural AR ligand (dihydrotestosterone, DHT) in the context of metastatic, CDK4/6 inhibitor (CDK4/6i) resistant breast cancer. Methods: Enobosarm and palbociclib treatments were evaluated in vitro by colony forming assays using CDK4/6i resistant (MCF7 PalbR) and both endocrine and CDK4/6i resistant (MCF7 cTamPalbR) cell lines. Next, enobosarm or DHT and palbociclib treatment were evaluated in vivo using endocrine and CDK4/6i resistant ER+ patient derived xenograft models (PDX) and cell line xenograft models. IHC, RNA and ChIP sequencing (AR, ER, H3K27ac) were subsequently performed on the harvested tumours. Results: While in vitro and in vivo growth of CDK4/6i resistant preclinical models was inhibited by treatment with a SARM or DHT alone, growth inhibition was more potent and durable in combination with a CDK4/6i. Gene set enrichment analysis of RNA-seq data integrated with ChIP-seq data revealed upregulation of an AR gene signature associated with a better prognosis following treatment with SARM. Co-treatment with a SARM and a CDK4/6i also enhanced AR signalling compared to SARM alone indicating an interaction of the two signalling pathways. Conclusion: Our data indicates that combination treatment with an AR agonist and a CDK4/6i represents a novel therapeutic strategy for CDK4/6i resistant ER+AR+ breast cancers

Citation Format: Allegra Freelander, Geraldine laven-Law, Leila Eshraghi, Nimmy Geetha, Peta Somerville, Marie Pickering, Sarah Alexandrou, C. Elizabeth Caldon, Wayne D. Tilley, Theresa E. Hickey, Elgene Lim. Selective Androgen Receptor Modulators in combination with CDK4/6 inhibitors demonstrate anti-cancer activity in preclinical treatment resistant ER+AR+ breast cancer models. [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-08-16.

Abstract PD2-02: Combination CDK4/6 inhibition and AR agonism suppresses the growth of CDK4/6 inhibitor resistant breast cancers

Resistance to standard-of care-therapies is a significant clinical challenge in estrogen receptor positive (ER+) breast cancer. Cyclin dependent kinase 4/6 inhibitors (CDK4/6i) in combination with endocrine therapies (ET) is the current standard-of-care for advanced metastatic ER+ breast cancer; however, resistance to this combination is considered inevitable, leading to disease progression. The androgen receptor (AR) is expressed in up to 90% of all ER+ breast cancers, and has been associated with better patient outcome. Compelling recent pre-clinical data demonstrates that selective androgen receptor AR modulators (SARMs) act to suppress ER-driven tumour growth of endocrine-sensitive and -resistant models of ER+ breast cancer. Furthermore, a recent clinical trial evaluating the efficacy of SARMs has shown clinical benefit in patients with ER+/AR+ metastatic breast cancer (NCT02463032). We hypothesise that the SARMS, either alone or in combination with a CDK4/6i, would be an effective treatment for tumours that are sensitive and resistant to ET and CDK4/6i. We report therapeutic efficacy of combination AR agonism + CDK4/6i in CDK4/6-naïve and -resistant pre-clinical models, including cell lines and patient derived xenograft (PDX) models. We demonstrate that combination SARM + CDK4/6i potently and durably inhibited in vitro and in vivo tumour growth. Additionally, we provide evidence that in vivo treatment with the CDK4/6i Palbociclib increased AR expression and signalling, highlighting an interaction of the two signalling pathways not previously described. In conclusion, our data provides a pre-clinical rationale for combination SARM + CDK4/6i in CDK4/6i resistant ER+ breast cancer.

Citation Format: Allegra Freelander, Geraldine Laven-Law, Leila Eshraghi, Kee Ming Chia, Marie Pickering, Aliza Yong, Ashleigh Wilkinson, Sarah Alexandrou, C. Elizabeth Caldon, Theresa E Hickey, Wayne D Tilley, Elgene Lim. Combination CDK4/6 inhibition and AR agonism suppresses the growth of CDK4/6 inhibitor resistant breast cancers [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD2-02.

Type 1 Nuclear Receptor Activity in Breast Cancer: Translating Preclinical Insights to the Clinic

The nuclear receptor (NR) family of transcription factors is intimately associated with the development, progression and treatment of breast cancer. They are used diagnostically and prognostically, and crosstalk between nuclear receptor pathways and growth factor signalling has been demonstrated in all major subtypes of breast cancer. The majority of breast cancers are driven by estrogen receptor α (ER), and anti-estrogenic therapies remain the backbone of treatment, leading to clinically impactful improvements in patient outcomes. This serves as a blueprint for the development of therapies targeting other nuclear receptors. More recently, pivotal findings into modulating the progesterone (PR) and androgen receptors (AR), with accompanying mechanistic insights into NR crosstalk and interactions with other proliferative pathways, have led to clinical trials in all of the major breast cancer subtypes. A growing body of evidence now supports targeting other Type 1 nuclear receptors such as the glucocorticoid receptor (GR), as well as Type 2 NRs such as the vitamin D receptor (VDR). Here, we reviewed the existing preclinical insights into nuclear receptor activity in breast cancer, with a focus on Type 1 NRs. We also discussed the potential to translate these findings into improving patient outcomes.